This pull requests adds a new CLI flag to `polkadot-parachains`
`--export-pov-to-path`. This CLI flag will instruct the node to export
any `PoV` that it build locally to export to the given folder. Then
these `PoV` files can be validated using the introduced
`cumulus-pov-validator`. The combination of export and validation can be
used for debugging parachain validation issues that may happen on the
relay chain.

`polkadot-parachain` simplifications and deduplications

Details in the commit messages. Just copy-pasting the last commit
description since it introduces the biggest changes:

```
    Implement a more structured way to define a node spec
    
    - use traits instead of bounds for `rpc_ext_builder()`,
      `build_import_queue()`, `start_consensus()`
    - add a `NodeSpec` trait for defining the specifications of a node
    - deduplicate the code related to building a node's components /
      starting a node
```

The other changes are much smaller, most of them trivial and are
isolated in separate commits.

Related to: https://github.com/paritytech/polkadot-sdk/issues/5

A couple of cosmetics and improvements related to
`polkadot-parachain-bin`:

- Adding some convenience traits in order to avoid declaring long
duplicate bounds
- Specifically check if the runtime exposes `AuraApi` when executing
`start_lookahead_aura_consensus()`
- Some fixes for the `RelayChainCli`. Details in the commits description